The present invention relates to an asymmetric arrangement of busbars for conducting the direct electric current from the cathode bar ends of a transversely disposed aluminum fused salt reduction cell to the anode beam of the next cell wherein a number of the busbars connected to the upstream cathode bar ends runs under the cell.
The production of aluminum via the fused salt electrolytic reduction of aluminum oxide involves dissolving the latter in a fluoride melt, the greater part of which is comprised cryolite. The precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the liquid aluminum itself forming the actual cathode in the process. Dipping into the melt from above are anodes which in the conventional processes are made of amorphous carbon. Oxygen is produced at the carbon anodes as a result of the electrolytic decomposition of the aluminum oxide; this oxygen combines with the carbon in the anodes to form CO.sub.2 and CO.
The electrolytic process takes place in a temperature range of approximately 940.degree.-970.degree. C. During the process the electrolyte becomes depleted in aluminum oxide. At a lower concentration of 1-2 wt.% aluminum oxide the anode effect occurs whereby the voltage rises from 4-5 V to 30 V and more. At this time at the latest the concentration of aluminum oxide must be raised by the addition of more alumina.
Embedded in the floor of the cell are cathode bars the ends of which protrude out of both sidewalls of the cell which are made up of a steel shell, insulation and carbon lining.
Energy losses of the order of up to 1 kWh/kg of aluminum produced are experienced as a result of the ohmic resistance in the stretch between the cathode bars and the anodes of the next cell. As a result, many attempts have been made to optimize the arrangement of the busbars with respect to ohmic resistance. In doing so, however, one must take into account the vertical components of the induced magnetic field which, together with the horizontal components of current density, produce field forces in the metal produced in the reduction process.
In an aluminum smelter with a series of transversely arranged reduction cells the current flows from cell to cell as follows: the direct electric current is collected by the cathode collector bars embedded in the carbon floor of the cell and leaves the cells, with respect to the general direction of current flow, at the upstream and downstream ends of these collector bars. The iron cathode bars are connected to aluminum busbars via flexible strips. The busbars, generally brought together as collector bars lead the direct current to the vicinity of the next cell where the current is conducted via other flexible strips and risers to the beam supporting the suspended anodes. Depending on the type of cell the risers are electrically connected to the end and/or one long side of the anode beam.
These busbars, characteristic for aluminum smelters, produce however disturbing effects both of an electrical and magnetic nature; attempts to eliminate these effects have been the subject of many publications up to now.
Disclosed in the British Pat. No. 1,032,810 in connection with an invention which is concerned with the hooding of cells is the proposal that the busbars can be arranged under the reduction cell. According to FIG. 2 of the British patent conductors 135 run, with respect to the transverse direction of the cell, symmetrically under the cell and are connected symmetrically to the anode beam of the next cell.
U.S. Pat. No. 3,415,724 aims at a conductor arrangement by which the magnetic effects are not increased when the current level is increased. To this end a part of the current leaving upstream from the cathode bar ends, but less than half, is conducted under the cell. The rest of the current leaving the cathode bar ends is led around the ends of the cell in a concentrated manner. As shown in FIG. 3 of the '724 patent the conductors leading the current under the cell lie at the middle of the cell and are in the form of collector conductor bars. The feeding of the current to the anode beam of the next cell is made at four points on the long side of the anode beam, symmetrical with respect to the transverse axis of the cell.
The disclosure of U.S. Pat. No. 4,313,811 is also drawn to an arrangement of conductor bars to conduct the direct electric current from the cathode bar ends of one transverse reduction cell to the anode beam of the next cell. The busbars connected to the upstream cathode bar ends are led alternately singly under the cell and in groups around the cell. The alternating groups comprise 1-5 conductor bars; preferably about a quarter of the total current is led under the cell.
Although, and in particular by means of the method in the last mentioned publication, the undesired magnetic and electrical effects can be largely eliminated, it is the object of the present invention to develop an arrangement of busbars for transverse fused salt aluminum reduction cells by means of which the investment costs and the current yield are optimized under conditions of practically negligible magnetic and electrical effects.